The present invention relates to superconductor integrated circuits and, more particularly, to an MoNx resistor for use in superconductor integrated circuit fabrication.
Superconductor integrated circuits include active devices, such as Josephson tunnel junctions, as well as passive devices such as resistors, capacitors, and inductors. Device operating parameters depend on the type of application in which the circuits are to be implemented. Varying the operating parameters of one of these devices often results in a need to change the parameters of one or more of the other devices.
The Josephson tunnel junction is the foundation of a superconductor integrated circuit. The relationship between the Josephson tunnel junction and the superconductor integrated circuit is analogous to the relationship between a transistor and a semiconductor integrated circuit. One characteristic of the Josephson tunnel junction is the critical current, which is the maximum superconducting current through the tunnel barrier.
Improved superconductor integrated circuit fabrication methods produce a superconductor integrated circuit with an increased critical current density. The increased critical current density results in a need for a resistor with a higher sheet resistance for certain circuit applications. Increasing the critical current density further requires a higher shunt resistance in parallel with the Josephson junction due to a smaller junction capacitance. Specifically, a resistor with a sheet resistance of approximately 3 ohms/square at 4xc2x0 Kelvin is required. In addition, the material used to form the resistor and the resistor fabrication process must be compatible with existing superconductor integrated circuit fabrication processes because the temperature and chemistry of resistor materials during resistor fabrication may adversely affect other devices on a superconductor integrated circuit wafer. Also, the process of fabricating the resistor must be easily reproducible for economic feasibility.
Molybdenum, palladium and other metals are used to form resistors in the fabrication of superconductor integrated circuits. However, the use of such metals results in resistivity values that are too low for high-speed circuit applications. A film thickness (resistor area) cannot be decreased to increase the resistance value of a resistor due to fabrication problems associated with very thin films. Specifically, because a silicon dioxide surface of a wafer on which a superconductor integrated circuit is formed is rough on an atomic level, the silicon dioxide surface may punch through the film forming the resistor or otherwise disrupt film growth if the film is too thin. In addition, a thin film may diffuse into the silicon dioxide by atomic diffusion. A very thick film of high resistivity should also not be used to form due to step coverage problems with layers of film deposited on top of the resistor.
Accordingly, an object of the present invention is to provide a resistor with a high sheet resistance for use in superconductor integrated circuit fabrication.
A further object of the present invention is to provide a superconductor integrated circuit resistor with a film thickness between 70 and 150 nm.
A further object of the present invention is to provide an easily reproducible method of fabricating a superconductor integrated circuit resistor in a manner consistent with existing superconductor integrated circuit fabrication processes.
Accordingly, the present invention provides a resistor with a high sheet resistance for use in superconductor integrated circuit (superconductor) fabrication, and a method of manufacturing the same. The superconductor includes a silicon substrate, a niobium ground layer, a first ground insulator layer formed from an anodized niobium and a second ground insulator layer formed from an oxide. A first resistor made of a chemical compound of molybdenum and nitrogen (MoNx) is provided between the first and second ground insulator layers, and a Josephson junction is provided above the first and second ground insulator layers. First and second oxide insulators isolate the various superconductor active and passive devices. Niobium interconnects provide electrical communication between an outer terminal and the various superconductor devices, as well as between elements of the superconductor circuit.
A method of fabricating the superconductor includes sputter depositing a niobium ground layer over a silicon substrate and subsequently reactive ion etching the niobium ground layer by photolithography. Next, niobium is anodized for forming a first ground insulator. MoNx is then sputter deposited over the niobium ground insulator layer and reactive ion etched by photolithography for forming a first resistor. A photoresist pattern is applied over the first resistor and first ground insulator. A niobiumxe2x80x94nitrogen compound is sputter deposited over the photoresist and the photoresist is subsequently lifted off the first resistor and first ground insulator layer for forming a second resistor. Silicon dioxide is sputter deposited over the first and second resistors and is then subsequently reactive ion etched by photolithography for forming a second ground insulator layer. A niobium layer is then sputter deposited over an aluminumxe2x80x94aluminum oxide compound which is then sputter deposited over another niobium layer and subsequently reactive ion etched by photolithography for forming a Josephson junction. A plurality of insulators and interconnects are then sputter deposited over the Josephson junction.
The superconductor provided according to the above method has a Josephson junction with a critical current density of 6 to 8 kA/cm and a first resistor sheet resistance between 3-5 ohms/square at 4xc2x0 Kelvin.